The present invention relates to the extractive metallurgy of refractory metals and in particular to tantalum and niobium refining. Tantalum refining can be defined as a process of impurity removal during metal consolidation by different sintering and melting techniques.
Oxygen has been considered as a major interstitial impurity in refractory metals, and the most traditional technique used to reduce its content has been deoxidization of refractory metal oxide within the metal (e.g. tantalum pentoxide) with carbon. The history of this method goes back into the beginning of the twentieth century when von Bolton obtained a patent (German Patent 216706, 1907) for producing ductile tantalum or niobium by adding carbon to the metal and heating the metal in vacuum, thereby removing carbon and oxygen as carbon monoxide. Rohn (German Patent 600369,1934) obtained a patent for production of niobium, and other refractory metals by adding a mixture of carbide and oxide to the molten metal pool while evacuating carbon monoxide.
Over the years the process for production of pure tantalum by carbon reduction of Ta.sub.2 O.sub.5 has been further developed and now comprises of vacuum sintering at 2000.degree. C. followed by one or two electron-beam melts to remove not only oxygen but also other impurities such as nitrogen, carbon, iron, nickel, silicon, and virtually all other elements, except for refractory metals such as W, Mo, and Nb. Four major drawbacks of this route are:
low yield (&lt;90% Ta, see example 1, below) due to sublimation of tantalum suboxides species; PA1 uncontrollable yield and productivity due to inhomogeneous (not crushed) scrap feedstock; PA1 low melting rate due to the high impurities level (basically C, N, Si, Fe, Ni, etc.); and PA1 tantalum carbide formation due to inhomogeniety of the feedstock PA1 help increase the rate of the carbothermic reaction; PA1 allow an increase of carbon/oxygen (C/O) ratio to stoichiometric value (5 mole/mole); PA1 result in lower residual O and C contents completely avoiding the formation of carbides (See Examples 1,2,3,4 and 5, below) and PA1 obtain representative samples and accurate analysis of impurities of the scrap.
Recently, Awasthi et al., (Journal of Alloys and Compounds, 1998, 265, 190-195), suggested substitution of silicon for carbon, in order to reduce tantalum losses. The experimental bench scale data demonstrated that in the temperature range of 1800-2000.degree. C. under 10.sup.-8 atm pressure and oxygen content &lt;0.1% oxygen removal by SiO (g) vaporization slows down and tantalum suboxide TaO (g) starts volatilizing which may amount to significant tantalum loss and is likely to decrease a melting rate due to silicon (silica) contamination.
The tantalum industry has been facing a challenge for some time to cope with the necessity of processing and refining significant amounts of scrap, generated primarily by the capacitor industry and containing more than 40 times as much oxygen and nitrogen as normal metallurgical grade powder. This causes high tantalum losses during pyrovacuum sintering and significantly slows down the melting rate in EB furnaces and usually calls for a second EB melting (See Examples 1 and 2, below).
It is a principal object of the present invention to provide a method of extraction of tantalum and other refractory metals from such scrap or other similar high oxygen sources.